The present invention relates to a thin film transistor (in general, called xe2x80x9cTFTxe2x80x9d) and a method of manufacturing the same, and particularly to a method of forming source and drain regions in a thin film transistor.
In recent years, there have been known active matrix liquid-crystal display unit using a thin film transistor. FIGS. 2A to 2D show a process of manufacturing a general thin film transistor. First, a silicon oxide film or silicon nitride is formed on a glass substrate 201 as a first coating film 202. A Corning 7059 glass or the like is used as a glass substrate. After the formation of the first coating film 202, a silicon semiconductor film which forms an active layer is formed on the first coating film 202. An amorphous silicon film is usually formed through the plasma CVD technique or low pressure thermal CVD technique, and thereafter the amorphous silicon film is crystallized by heating or the application of laser beam. Then, a silicon film subjected to a crystal property (hereinafter referred to as xe2x80x9ccrystalline silicon filmxe2x80x9d) is patterned to thereby form an active layer 203. (FIG. 2A).
After the formation of the active layer 203, a silicon oxide film is formed as a gate insulating film 204 through the plasma CVD technique or the sputtering technique. Then, a gate electrode 205 is formed of material mainly containing metal or semiconductor. After the formation of the gate electrode 205, impurity ions are injected thereinto so as to form a source region 207 as well as a drain region 209. This process is executed using the gate electrode 205 as a mask. As the ions injected, P (phosphorus) is used in the manufacture of an n-channel thin film transistor, whereas B (boron) is used in the manufacture of a p-channel thin film transistor. Also, a channel formation region 208 is formed simultaneously during this process. (FIG. 2B).
After the formation of the source region 207 and the drain region 209 as well as the channel formation region 208, the source region 207 and the train region 209 are recrystallized by application of a laser beam or an infrared ray, and the impurity ions injected into those region are activated. The recrystallization of the source region 207 and the drain region 209 are made because the source region 207 and the drain region 209 have been made amorphous by the bombardment of injected ions at the time of the preceding ion injection.
The above-mentioned recrystallization and activation of the source and drain regions may be performed by heating. However, in the case of heating, its effect could not be obtained without heating at temperature of 700xc2x0 C. or higher (preferably 800xc2x0 C. or higher). Taking the heat-resistivity of a glass substrate (a substrate made of Corning 7059 glass must be dealt with at 600xc2x0 C. or lower) into account, such a heat treatment is improper.
Subsequently, an interlayer insulating film 211 is formed of silicon oxide or other insulating materials. Further, after forming contact holes, a source electrode 212 and a drain electrode 213 are formed of a proper metallic material.
The thin film transistor manufactured through the foregoing processes suffers from such a problem that its characteristics are deteriorated or largely dispersed. This problem results from the fact that defects concentrate in the vicinity of interfaces between the source region 207 and the channel formation region 208 and between the drain region 209 and the channel formation region 208.
In other words, the source region 207 and the drain region 209, which have been made amorphous by the injection of ions in the process of FIG. 2B, are recrystallized by the application of a laser beam in the process of FIG. 2C, during which the channel formation region 208 remains crystalline. Therefore, the crystallization of the source and drain regions, which progresses by the application of a laser beam, stops at the interfaces between the source and drain regions and the channel formation region having the crystal property from the first. As a result, a large number of defects resulting from mismatching of lattices are produced in the vicinity of the interfaces between the source and drain regions and the channel formation region. The existence of those defects makes not only the characteristics dispersed and unstable but also an off-state current increase.
As a manner of solving the foregoing problem, it has been found that the recrystallization of the source and drain regions and the activation of the impurity ions are performed at a temperature of 700xc2x0 C. or higher, preferably 800xc2x0 C. or higher. If the recrystallization of the source and drain regions and the activation of the impurity ions are performed at a temperature of 700xc2x0 C. or higher, preferably 800xc2x0 C. or higher, energy is also applied to the channel formation region 208. Hence, mismatching of lattices produced in the vicinity of the interfaces between the source and drain regions and the channel formation region can be released, as a result of which the defects can be prevented from concentrating in the vicinity of the interfaces between the source and drain regions and the channel formation region.
However, in order that processes for the recrystallization of the source and drain regions and the activation of impurity ions injected are performed by a process of heating at 700xc2x0 C. or higher, a substrate capable of resisting a temperature of 700xc2x0 C. or higher must be used. However, such a substrate is expensive, resulting in a large obstacle to the use of the thin film transistor in a liquid-crystal display apparatus. In other words, in the use of an inexpensive glass substrate having a heat-resistant temperature of 600xc2x0 C. or lower, the processes for the recrystallization of the source and drain regions and the activation of impurity ions cannot be realized by heating for all practical purposes.
The present invention has been made in view of the above problems with the prior art, and an object of the invention is to execute the recrystallization and activation of source and drain regions at a temperature lower than that in the prior art.
Another object of the invention is to provide a thin film transistor with a structure in which defects on interfaces between source and drain regions and a channel formation region are reduced.
A still another object of the invention is to provide a thin film transistor which is sufficiently high in a crystal property of the source and drain regions.
In order to achieve the above objects, according to one aspect of the invention, there is provided a method of manufacturing a thin film transistor, comprising the steps of:
introducing a metal element for promoting crystallization into an amorphous silicon film;
subjecting said amorphous silicon film to a heat treatment to form a crystalline silicon film;
forming an active layer using said crystalline silicon film;
selectively injecting impurity ions into a part of said active layer; and
subjecting said active layer to a heat treatment to grow crystal from a region into which said impurity ions have not been injected toward a region into which said impurity ions have been injected.
In the foregoing structure, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag and Au are applicable as the metal element which promotes crystallization. In particular, the use of Ni (nickel) makes it possible to obtain a remarkable effect.
An amorphous silicon film is formed on a glass substrate, a quartz substrate, or a semiconductor substrate or metal substrate having an insulating surface. The amorphous silicon film is formed through a vapor phase technique such as a plasma CVD technique or low pressure thermal CVD technique, or the sputtering technique.
As the impurity ions, ions of phosphorus or boron are used.
In the foregoing structure, it is effective to apply a laser beam or an intense light beam to the formed film before or after the heat treatment. In particular, the application of a laser beam after the heat treatment makes the crystal property of the film effectively enhanced.
According to another aspect of the invention, there is provided a method of manufacturing a thin film transistor, comprising the steps of:
introducing a metal element for promoting crystallization into an amorphous silicon film;
subjecting said amorphous silicon film to a heat treatment to form a crystalline silicon film;
forming an active layer using said crystalline silicon film;
selectively injecting impurity ions into regions of said active layer; and
subjecting said active layer to a heat treatment to recrystallize the impurity doped regions with the region which is not introduced with the ions used as crystal nuclei.
According to still another aspect of the invention, the active layer has such a structure that a crystal grows from said channel formation region toward the source and drain regions adjacent to said channel formation region with the channel formation region functioning as a crystalline nucleus.
For example, in a thin film transistor as shown in FIG. 3D, a crystal growth develops as indicated by arrows 301 with a channel formation region 208 being a crystalline nucleus as shown in FIG. 3C, as a result of which a source region 207 and a drain region 209 are crystallized.
According to yet still another aspect of the invention, the active layer has such a structure that a crystal grows from a peripheral region of the channel formation region toward the source and drain regions adjacent to said peripheral region with said peripheral region being a crystalline nucleus.
For example, in a thin film transistor shown in FIG. 1D, a crystal growth develops from an offset gate region 108 which is a periphery region of the channel formation region, as a result of which a source region 107 and a drain region 110 are crystallized in a process of FIG. 1C.
As said peripheral region of the channel formation region, an offset gate region, a light doped region and a non-doped region each having a crystal property without being subjected to ion injection are applicable.
As a metal element which promotes crystallization useable to the present invention disclosed in this specification, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag and Au, which are elements interstitial with respect to silicon, are usable. Atoms interstitial with respect to silicon are dispersed into a silicon film during a heat treatment. While the interstitial elements are dispersed thereinto, the crystallization of silicon progresses simultaneously. In other words, the interstitial metal makes the crystallization of an amorphous silicon film promoted with a catalytic action of the dispersed metal.
The introduced amount (added amount) of the interstitial elements becomes important since the interstitial elements are allowed to be rapidly dispersed in the silicon film. In other words, the small introduced amount of the interstitial elements makes the effect of promoting crystallization low so that an excellent crystal property cannot be obtained. On the other hand, the excess amount of the interstitial atoms makes the semiconductor characteristic of silicon spoiled.
Therefore, there exists an optimum amount of introducing the interstitial metal elements into the amorphous silicon film. For example, it has been found that, in the case of using Ni as a metal element which promotes the crystallization, if the density of Ni in a crystallized silicon film is 1xc3x971015 cmxe2x88x923 or more, the effect of promoting crystallization can be obtained, whereas, if the density of Ni in the crystallized silicon film is 1xc3x971019 cmxe2x88x923 or less, the semiconductor characteristic is not hindered. The xe2x80x9cdensityxe2x80x9d used here is defined by the minimum value obtained through SIMS (Secondary Ion Measurement System). Also, the above recited metal elements other than Ni can obtain their effects in the same density range as that of Ni likewise.
Apart from the above recited metal atoms, the use of Al or Sn can make the crystallization of an amorphous silicon film promoted likewise. However, Al or Sn causes an alloy to be formed in combination with silicon so as not be dispersed and interstitial into a silicon film. Then, the crystallization progresses in such a manner that a crystal growth develops from a portion where Al or Sn is alloyed with silicon with that portion being a crystalline nucleus. In this case, as a result that Al or Sn is not dispersed into the silicon film, the crystallization progresses from the portion of the crystalline nucleus. In this way, in the use of Al or Sn, a crystal growth is conducted only from a portion where Al or Sn is introduced (that is, an alloy layer consisting of those elements with silicon). This causes a problem such that its crystal property is generally lowered in comparison with a case of using the foregoing interstitial elements such as Ni. For example, there arises such a problem that this makes it difficult to obtain a crystalline silicon film which is uniformly crystallized.
As a result that a metal element promoting the crystallization is introduced and a semiconductor layer with a region having a crystal property as well as a region having an amorphous property is subjected to a heat treatment, the amorphous region can be recrystallized with the crystalline region being a crystalline nucleus. In this situation, since the crystallization progresses from the crystalline region toward the amorphous region, mismatching of lattices is not produced in the vicinity of the interfaces between the crystalline region and the amorphous region, thereby being capable of preventing the defects from concentrating.
For example, a silicon semiconductor layer is crystallized by a metal element which promotes crystallization and includes a specified region which has been made amorphous with the injection of impurity ions giving one conductivity type thereinto. Such a silicon semiconductor layer is subjected to a heat treatment so that crystal growth progresses from the region having the crystal property toward the region which has been made amorphous by the injection of impurity ions giving one conductivity type. Thus, the region which has been made amorphous can be crystallized.
In this situation, since the crystallization progresses from the region having the crystal property toward the region having the amorphous property, defects resulting from mismatching of lattices can be prevented from being produced in the vicinity of the interfaces between the respective regions. Thus, the semiconductor layer having the crystal property where the defects do not concentrate in a specified region can be obtained.
Also, in a process of manufacturing a thin film transistor including a manufacturing process shown in FIGS. 3A to 3D, the active layer 203 is constituted by a silicon film which has been crystallized by the heat treatment due to the action of the metal element which promotes crystallization. During the process of FIG. 3B, impurity ions giving one conductivity type are injected into such an active layer 203 with a mask of the gate electrode 205 so that the source region 207 and the drain region 209 are formed in a self-aligning manner. When the source and drain regions have been made amorphous, a heat treatment is conducted at 550xc2x0 C. for about 2 hours in the process of FIG. 3C. As a result, the crystal growth is made, as indicated by the arrows 301, toward the source and drain regions with the channel formation region 208 having the crystal property functioning as a crystalline nucleus.
The crystal growth indicated by the arrows 301 is conducted at a relatively low temperature for a short period of time with the channel formation region 208 functioning as the crystalline nucleus with the following causes.
(1) A catalyst element which promotes the crystallization of silicon has been introduced into the active layer 203.
(2) The source region 207 and the drain region 209 have been doped with an element having one conductivity type which is of a catalyst element for promoting crystallization.
Since the foregoing crystal growth is conducted from the channel formation region toward the source and drain regions as indicated by the arrows 301, the defects resulting from the mismatching of lattices do not concentrate in the vicinity of the interface between the channel formation region 208 and the source region 207, as well as in the vicinity of the interface between the channel formation region 208 and the source region 209.
The above and other objects and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings.